The present invention concerns an opto-electronical electronic apparatus which can be used to check or to measure directly, without calculations or additional corrections, the orientation of light beams and their luminous intensity, in one or several characteristic directions by setting up the apparatus at any distance in front of the light beam emitter.
One of the various applications of the invention is to measure or to check the orientation and the luminous intensity of projectors or signalling lights either fixed or mounted on vehicles.
The traditional method to measure the orientation of a light beam consists in observing its shape and its position on a screen set up at a large distance in front of the emitter. The luminous intensity I in a given direction can be calculated on the basis of the illumination E (in lux) on the screen at the place corresponding to this direction and can be expressed as follows: EQU I=E.R.sup.2
where R (in meters) is the distance of the screen with regard to the emitter.
In order to make these operations more practical, apparatus containing a converging lens are known. This lens reproduces the image of the light beam on a screen put close to its focal plane. The image obtained is similar to the image on a large distance screen.
In particular, such optical apparatus for car headlights have been standardized in several countries, one of which is Belgium (standard NBN No. 612).
The luminous intensity I in a given direction can then be calculated in function of the illumination E of the corresponding point on the screen and is expressed as follows: EQU I=E.F2
where F (in meters) is the focal distance of the lens.
A photo-electric transducer placed on the screen allows one to measure the luminous intensity in the corresponding direction whereby the measure is independent of the distance to the emitter in so far as all the rays coming from the emitter in this direction are received by the lens.
By placing several photo-electric transducers on the screen, it becomes possible to compare the luminous intensities in different directions and consequently to measure the orientation of a light beam.
The following comparisons are known up to now:
For symmetrical light beams with regard to two perpendicular planes (the intersection of which makes the light beam axis): four photo-electrical cells, one in each quadrant, may be placed on a movable screen with regard to the light beam (or on a immovable screen with regard to the light beam reflected by means of a movable mirror e.g.). Detecting the balance between the electrical currents generated by the four cells by means of zero detectors allows one to measure the orientation of the light beam.
For light beams presenting a fast gradient of the luminous intensity in the vicinity of a geometric place that we will call "cut-off", the photo-electrical cells may be placed in the neighborhood of this cut-off and:
either the difference of illumination between these cells can be measured;
or the illumination of two cells placed like this can be compared to a linear equation: EQU I.sub.2 =aI.sub.1 +b
where I.sub.1 and I.sub.2 are the luminous intensities in the corresponding directions 1 and 2, a and b being predetermined coefficients that are finite and not equal to zero.
These realizations have the following inconveniences:
either they require that the operator carry out a measure and then one cannot tell immediately and clearly whether the orientation of the light beam is within the tolerances of not,
or they give a result not only depending on the orientation of the light beam but also on the absolute values of the luminous intensities. In practice, these values are often variable, among other things because of the supply voltage variations of the lamps.
Besides, it is not possible to check whether the shape of the light beam is satisfactory or not.
Already for a long time the need has been felt to measure or to check in a direct and easy way the shape and the exact orientation of different light beams.
The best known example is that of the car headlights and more precisely that of the driving lights and the passing lights. Indeed, according to the 1968 Vienna Convention on Road Traffic:
the driving lights must be capable to illuminate the road adequately over at least 100 m ahead of the vehicle,
the passing lights must be capable to illuminate the road adequately over at least 40 m ahead of the vehicle, without causing undue dazzle or inconvenience to on coming drivers and other road-users.
That is why standards have been laid down:
for the "American" headlights: SAE standards
for the "European" headlights: Rules annexed to the 1958 Geneva Agreements concerning the adoption of uniform Conditions of Approval for Motor Vehicle Equipment and Parts.
These standards prescribe minimum luminous intensities in the directions to be illuminated and maximum luminous intensities in the directions in which dazzle is to be feared. These luminous intensities must be measured in well-defined laboratory conditions during type approval tests on headlights.
These measures cannot be carried out on vehicles up till now by lack of adequate instruments. Indeed, such instruments must be adapted at the practical working conditions as they are, for example, in the inspection stations for motor vehicles.
Consequently, one of the objects of the present invention is to provide a measuring--and checking--apparatus for motor vehicle headlights for allowing one to ascertain immediately whether a headlight fulfils the requisite conditions or not, being it understood that the application is not limited to this specific case.